A whispering-gallery-mode (WGM) microcavity is a tiny optical resonator, often a glass sphere tens of micrometers across, in which light circulates repeatedly around the curved surface. In the reported device, an ultrasound pulse slightly deforms the microsphere and shifts its refractive index, which modulates the optical mode and encodes the ultrasound signal onto the transmitted optical signal, from which amplitude and frequency information can be extracted by demodulation. That physical principle has long made WGM resonators attractive as acoustic sensors, but turning laboratory prototypes into practical probes has been difficult: the fragile microsphere, its coupling fiber and the supporting hardware typically add up to a sensing head several millimeters wide, far too bulky for tight spaces such as blood vessels or for scanning across uneven surfaces at close range.
A team reporting in Frontiers of Optoelectronics has now packaged a silica WGM microsphere into a needle-like probe roughly 2 millimeters in diameter that works in both air and liquid. The device, described in a research article published on 14 August 2025, achieves optical quality factors up to 2 × 10^7, a noise equivalent pressure of 5.4 mPa/√Hz for ultrasound detection, a −6 dB acoustic bandwidth exceeding 40 MHz, and the ability to record the vibration spectra of microparticles at frequencies reaching a few hundred megahertz. The authors argue that the combination of sensitivity, ruggedness and small size could carry WGM sensing out of the cleanroom and into applications such as photoacoustic endoscopy and near-field ultrasound inspection.
The central engineering challenge was coupling light into the microsphere and keeping that coupling stable under real-world handling. The new design uses a side-coupled geometry: a tapered fiber is drawn down to about 1 micrometer in diameter by hydrogen-flame tapering, then bent into a U-shape by precision axis rotation and fixed with UV-cured adhesive. The folded tip of this fiber is bound directly to the surface of the roughly 70-micrometer silica microsphere, exploiting the spreadability and molecular surface tension of polymer microdroplets on the glass to hold the taper in place. The fiber runs parallel to the microsphere’s stem, so the whole assembly stays compact.
Encapsulation is the second half of the trick. A thin layer of low-refractive-index polymer droplets seals the coupling region, protecting it mechanically while remaining acoustically transparent so that ultrasound still reaches the cavity with high transmission efficiency. Outside that, a double-layer shell of glass and aluminum alloy reinforces the device, and a standardized FC/APC fiber port makes it plug-and-play. The authors report that after full encapsulation they can maintain an optical transmittance of more than 20 percent through the folded taper, despite losses introduced during tapering, folding, adhesive dispensing and sealing. Fabrication demands care: a five-dimensional translation stage is needed to control the angle and relative position of the microcavity to the fiber, and the team monitored transmittance continuously through the U-shaped fiber preparation and coupling process.
Characterization with a tunable laser spanning 910 to 980 nanometers revealed the microcavity’s resonance modes, and analysis of a single mode yielded a maximum quality factor of 2 × 10^7. That figure matters because the Q factor determines how sharply the resonance responds to perturbation: a higher Q means the optical energy circulates longer and the light-matter interaction is intensified, which directly translates into greater acoustic sensitivity. The small cavity volume also concentrates pump light into a very high energy density, a second contributor to the probe’s performance. Because the coupling structure is sealed, the same device can be immersed in water or other liquids without degrading the resonance—a prerequisite for the photoacoustic imaging demonstrations that followed.
To quantify ultrasound sensitivity, the researchers measured the probe’s response to 20 MHz ultrasound waves emitted by a transducer driven by a pulse/echo receiver, calibrating against an ONDA HGL-1000 hydrophone under identical conditions using a procedure from their earlier work. After band-pass filtering across 1.2 to 800 MHz, the detected signal yielded a noise equivalent pressure of 24 Pa, equivalent to 5.4 mPa/√Hz. For comparison, the authors cite a needle hydrophone with a noise equivalent pressure of 768 Pa from established research. The frequency response was assessed by firing nanosecond pulses from a pulsed laser—about 1.8 ns pulse width and roughly 2 microjoules of pulse energy—at a 50-nanometer copper film, which generates a broadband photoacoustic signal. Fourier analysis of the detected time-domain response showed a bandwidth exceeding 40 MHz at −6 dB, a behavior the team attributes to the acoustic properties of the cavity material and to how ultrasound reflects at the cavity, with the integrated cavity-and-stem arrangement guiding sound through and away so that multiple internal reflections are suppressed.
Directionality was also characterized. The probe’s response stays within −6 dB over an angular range of 180 degrees. In principle a microsphere acts as a point detector responding uniformly over the full space angle, but the protective shell can block some incoming ultrasound; the authors note that repositioning the shell allows the receiving angle to be tailored to a given application. With these performance figures established, the team built a photoacoustic microscopy system in which pulsed light is focused by an objective lens onto the sample from below while the microcavity probe, immersed in water alongside the sample, captures the emitted photoacoustic signal from above during two-dimensional scanning. Ultrasound amplitudes are extracted from time-domain pulse peaks after band-pass filtering.
The imaging results span several test targets. Tightened hairs fixed to a glass slide appear with their contours resolved and even the scaly surface structure visible. Two curly hairs buried in a hydrogel were imaged with high contrast. A gold film engraved with letters roughly 50 micrometers wide showed clear outline edges, and an ant immobilized in hydrogel yielded a well-defined silhouette, with one bent leg outside the pulsed-light focus remaining indistinct. Each image took no more than 15 minutes. Lateral resolution was calibrated by scanning a blade edge: the derivative of the transition signal’s line spread function gave a full width at half maximum of about 2 micrometers, optimized through adjustments of the numerical aperture and beam diameter, following standardized photoacoustic-imaging calibration methodology.
Beyond imaging, the probe doubles as a contact sensor for mesoscopic vibrational spectroscopy. The microsphere is surrounded by a polymer glue layer several micrometers thick, which fixes the coupling point without blocking high-frequency ultrasound. When 532-nanometer pulsed light is focused onto particles adhering to the probe, the resulting broadband photoacoustic signal spans the particles’ vibrational spectrum; resonant frequencies are enhanced while other frequencies rapidly attenuate. Detected electrical signals were amplified by 26 dB over 40 kHz to 38 GHz and recorded with an oscilloscope using 64-fold averaging. For polystyrene particles with a radius of about 2.8 micrometers, vibrations persisted for more than 100 nanoseconds, with a main peak at 355 MHz—a value consistent with theoretical predictions from the natural-frequency formula for elastic spheres using the material’s density of 1.05 g/cm³, Young’s modulus of 3.24 GPa and Poisson ratio of 0.34. The authors stress that this continuous acoustic sensing differs from the pulsed-signal bandwidth used in photoacoustic imaging, and that with sufficient sensitivity the microcavity can measure vibrations well below the −6 dB point; they state that the upper frequency limit for vibrational spectroscopy with their microprobe can exceed the gigahertz level, governed by the cavity’s Q factor and operating frequency.
The authors position the device as a step from laboratory demonstration toward practical deployment, citing potential uses in photoacoustic endoscopic imaging, gas and liquid photoacoustic spectroscopy, and metal flaw detection. Compared with microcavities fabricated on chips, they argue, the packaged sphere offers a simpler package, greater application flexibility and stronger ultrasound response, and adjusting the microsphere size can tune the measurement to different center frequencies and bandwidths. The work remains a laboratory demonstration: the reported sensitivities, imaging results and vibration spectra were all obtained under controlled experimental conditions, and the fabrication process still depends on precise manual coupling with a five-dimensional translation stage. Even so, the combination of a 2-millimeter probe head, quality factors of 10^7, sensitivity at the millipascal-per-root-hertz level and operation in both gaseous and liquid environments marks a notable advance in making whispering-gallery-mode ultrasound sensing compact, robust and usable outside the benchtop.
Subject of Research: A compact silica whispering-gallery-mode microprobe for high-sensitivity ultrasound detection, photoacoustic imaging, and mesoscopic vibration spectroscopy
Article Title: WGM microprobe device for high-sensitivity ultrasound detection and vibration spectrum measurement
Article References: Sun, J., Huangfu, S., Chen, T., Cai, Z., Ruan, B., & Zhang, F. (2025). WGM microprobe device for high-sensitivity ultrasound detection and vibration spectrum measurement. Frontiers of Optoelectronics, 18(1), Article 17. https://doi.org/10.1007/s12200-025-00161-7
Image Credits: AI Generated
DOI: 10.1007/s12200-025-00161-7
Keywords: whispering-gallery-mode, microcavity, ultrasound detection, photoacoustic imaging, vibration spectroscopy, microsphere, Q factor, NEP, optical sensing, endoscopy, microprobe, device
News Source: Denise Maddox. (October 29, 2026). Research report: WGM microprobe device for high-sensitivity ultrasound detection and vibration spectrum measurement. Scienmag.
